Analytical system



Get. 30, 1945. H, w H N 2,387,786

ANALYTICAL SYSTEM Filed July 20, 1942 2 Sheets-Sheet l ivolv BEAM DEFLECT/ON CONTROL CIRCUIT MAGNET. 3/

P/RANI GAUGE T0 VACUUM v PUMP SAMPLE IN TAKE was 1' M v a E A 2/ 29 i 7?: 5 25 7 ro l/ACUUMPUMP INVENTORT IMROLD m WASHBURN.

A N BY 3 MN 56 55 W4; ARI-NT.

Uc. 39, 1945. w, w s uR 2,387,786

ANALYTICAL SYSTEM Filed July 20, 1942 2 Sheets-Sheet 2 INVENTOR, HAROLD n! WASHBURN.

} AGENT.

Patented Oct. 30, 1945 ANALYTICAL SYSTEM Harold W. Washburn, Pasadena, Calil'., assignor Consolidated Engineering Corporation,

Pasadena, Calif., a corporation of California Application July 20, 1942, Serial No. 451,664

17 Claims.

My invention relates to mass spectrometry and more pa ticularly to a gas inlet system ideally adapted for simultaneously increasing sensitivity or recording speed or both while maintaining linear superposition in the analysis of mixtures with a mass spectrometer.

In the analysis of a gas mixture with a mass spectrometer, the gas is admitted from a sample region into an ionization region where the molecules of gas are ionized such as by electron bombardment, and the ions formed there are withdrawn into an analysis region where ions of different mass-to-charge ratios are segregated into beams and successively focused upon an ion collector. The measured peak intensities of the ion beams represent the amounts of the Withdrawn ions or the rates of formation of the respective ions. Such a set of measured peaks is a mass spectrum.

In order to facilitate the analysis of the mass spectrum of a mixture, each peak occurring in the mixture spectrum should represent the sum of the corresponding peaks that would be obtained in mass spectra of the separate components if present alone. linear superposition. When linear superposition is maintained, the composition of the mixture under investigation may be determined relatively simply by the analysis of a set of linear simultaneous equations which involve 1) the intensities :1

of the peaks of beams of ions of different massto-charge ratio occurrin in the spectrum of the mixture, (2) the unknown quantities of the dif ferent components in the mixture, and (3) constants representing the sensitivity or efiiciency of the mass spectrometer in producing ions of the various mass-to-charge ratios from a unit quantity of each of the components.

It is possible to obtain linear superposition by maintaining the sample in a sample chamber homogeneous at all times during analysis, flowing the components from the sample chamber into an ionization chamber through a gas inlet at mutually independent rates, ionizing each component in proportion to its partial pressure in the ionization chamber and independently of the amounts of other components there, and providing such pressure conditions in the mass spectrometer that collisions between ions withdrawn from the ionization chamber with any molecules either in the ionization chamber or in the analyzing chamber are relatively infrequent. Preferably the inlet is in the form of an orifice and the flow is maintained by the differential pressure on the two sides of the orifice. In order to main- This efiect is known as tain flow of the individual components from the sample chamber into the ionization chamber through the orifice mutually independent, the pressure in the sample chamber is maintained low enough so that the mean free path of the molecules in the sample chamber is larger than the radius of the orifice connecting the sample chamber to the ionization chamber. In such a case, the frequency of intermolecular collisions in the orifice is negligible compared with the frequency of collisions of the molecules with the orifice wall.

In the practice of the foregoing method, I have found that when the pressure of the gas sample in the sample chamber is reduced to the point where practically perfect linear superposition occurs, and analysis of mixtures to an accuracy of about i- 0.5 of 1% becomes possible, the reduction in pressure in the ionization chamber causes a substantial reduction of the sensitivity of the mass spectrometer. To compensate for this reduction in sensitivity the resistance through which the ions are discharged may be increased. However, there is a practical limit above which it is not desirable to increase the resistance due to the resultant increase in electrical noise. Furthermore, such an increase in resistance has the further disadvantage that it may cause a substantial increase in the time required to record a mass spectrogram.

In order to overcome the above limitations of this prior method, according to the instant invention, I connect the sample bottle to the ionization chamber through an inlet system comprising a plurality of inlets each of which acts to flow each component present in the gas mixture into the ionization chamber at a mutually independent rate and maintain the rate at which gas flows out of the ionization chamber as low as possible, thus producing a substantial increase in the gas pressure in the ionization chamber. In the preferred form of my invention, the inlet system comprises a plurality of orifices of small dimensions providing a plurality of paths in parallel, the word parallel being employed in the electrical rather than the geometric sense. Since all the orifices of such an inlet system admit gas simultaneously into the ionization chamber, the sensitivity of the mass spectrometer is increased in direct proportion to the number of such orifices once a suitable sample chamber pressure has been established for achieving linear superposition with an orifice of given dimensions.

The principal object of my invention is to provide a new means and method for achieving linear superposition in mass spectrometry.

Another object of my invention is to provide a means and method for increasing the sensitivity or the recording speed, or both, of a mass spectrograph while maintaining linear superposition.

Still another-object of my invention is to provide a simple method of making a small orifice for use as a mass spectrometer inlet.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way. to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

My invention may be more readily understood by direct reference to the drawings in which:

Fig. 1 shows a general organization of a mass spectrometer incorporating one form of my inlet system;

Fig. 2 is a schematic drawing showing a section taken on the line 2-2 of Fig. 1 of part of the mass spectrometer including the ionization chamber;

Fig. 3 is a perspective drawing, partly in section, of one preferred form of my inlet system;

Fig. 4 is a detail perspective drawing of a single orifice of the form I prefer to use in the inlet system of Fig. 3;

Fig. 5 is a perspective drawing of a preferred type of orifice in the form of a thin glass disk ground from a piece of capillary tubing;

Fig. 6 is a sectional drawing of a part of an orifice at one stage of manufacture;

Fig. 7 represents an inlet system in the form of a disk containing a plurality of orifices passing therethrough which may be utilized in my invention.

In Fig. 1, I have shown a sample chamber I connected through an inlet system 2, to an ionization chamber 3 of a mass spectrometer.

The sample bottle I is connected at one end to said inlet system through a high vacuum right-angle stopcock 5, and at the other end to a sample intake connection I and to a vacuum pump connection 9 through stopcocks II and I3 respectively. The sample chamber is also provided with a pressure measuring device I5, such as a Pirani gauge.

To prepare the apparatus for the'analysls of a sample. stopcocks 5 and I3 are opened and stopcock II closed, and the sample chamber I exhausted by evacuation. Stopcocks 5 and I3 are then closed and a sample introduced from the sample intake connection I by opening stopcock I I, which is then closed.

In the event that the pressure of the sample to be analyzed in the sample chamber is higher than a suitable predetermined value, said pressure may be reduced by a number of methods such as by exhausting part of the sample through the vacuum pump connection 9.

When the sample pressure is at a suitable value, and the remaining portions of the apparatus are in a ready condition, the high vacuum right-angle stopcock 5 is opened, and the sample slowly introduced into the ionization chamber 3 through inlet system 2.

As illustrated in Figs.-1 and 2, electrons emitted from a heated filament I I in the ionization chamber are directed in a beam I8 through apertures 20-20 in the wall of said ionization chamber 3, along a line perpendicular to the face of a magnetic pole I! by the combined action of a magnetic field indicated by thearrow 22 in Fig.

2 and which is directed downward perpendicular to the plane of Fig. 1, and an electric field parallel to the magnetic fleld provided by a battery (not shown). These electrons bombard and ionize molecules of gas in the ionization chamber.

Positively charged ions formed in the ionization chamber by collision with the electrons are accelerated toward first slit electrode 2i by action of a, small electric potential which maintains said first collimating slit electrode negative with respect to a pusher electrode 22 on the opposite side of the electron beam. Some of the accelerated ions pass through a narrow slit 24 in said first collimating slit electrode 2| and are thereupon accelerated by a large negative potential maintained between second ccllimating slit electrode 25 and said first colllmating slit electrode 2|. some of the accelerated ions then pass through a second slit 2! in said second collimating electrode.

Owing to the combined action of said accelerating field and the magnetic field, positive ions passing through said second collimating slit 2! follow circular paths, and ions of a predetermined mass-to-charge ratio are focused at exit slit 21, positioned in front of ion collector 2!. Said exit slit 2'! is positioned on the circumference of a semicircle 3| in the center of analyzer 23 and located from the mid-point between collimating slits 24 and 26.

The pusher electrode 23, first collimating slit electrode 2 I, and the second collimating slit electrode 25 are insulated from each other and connected to suitable points of a potentiometer H in ion beam deflection control circuit 43.

In order to detect ions of different mass-tocharge ratios formed in the ionization chamber, a charge which has been accumulated on a condenser 45, connected across said potentiometer 4| is allowed to discharge /through said potentiometer when a key 41 is opened. As the charge on said condenser 45 changes, the ion accelerating potentials existing between said pusher electrode 23, said first collimating slit electrode 2I and said second collimating electrode 26 is reduced exponentially and beams comprising ions of larger and larger mass-to-charge ratio are swept past the exit slit 2! before collector 24. As these beams are swept past the exit slit 21, ions in the different beams are successively discharged at the ion collector 29 through a resistance 50, and the resultant ion collector currents successively actuate a recorder 5| having resistance 50 in its input and thereby produce a mass spectrogram 53 of the sample. Said mass spectrogram is preferably in the form of a trace 54 in which successive displacements thereof represent the intensities of beams successively detected by the ion collector 29. The deflection control circuit and recording system are described in more detail in copending patent application, Serial No. 444,491, filed May 25, 1942.

In the preferred form of my invention, the inlet system 2 connecting the sample chamber and the ionization chamber is provided with a plurality of small apertures or orifices through each of which different components of the gas sample are passed at mutually independent rates. As a further aid in obtaining increased sensitivity, said ionization chamber is completely closed except for slit 24 and apertures 20-20. The cross-sectional area, of said apertures is made as small as possible and preferably less than the cross-sectional area of slit 24. The rate of pumping gas out of said ionization chamber is thus made as small as possible and is governed primarily by the resistance to pumping provided by said slit 24. Gas passing out of apertures 20 is evacuated through pumping line 55 connected to an airtight case 56 surrounding the ionization chamber; and gas passing out of the slit 24 may pass into the analyzer 33 through slitJB or side apertures 51-51 in collimating slit electrode 25 and thence evacuated through pumping line 58 connected to the analyzer. While the widths of the two slits 24 and 26 may be the same, slit 24 is preferably made narrower.

As illustrated in Fig. 3, in its preferred form my inlet system comprises a connector having two open ends GI and 63 adapted for connection to a sample chamber and to an ionization chamber respectively and a wall 65 separating said ends and having a plurality of orifices 61 therein. In the form shown here, the connector comprises a piece of glass tubing 69 having one end 63 open and the opposite end closed by wall 65 except for the plurality of orifices 61. A second piece of glass tubing H of larger diameter than tubing 69 is fused toa circumferential raised portion 13 in tubing 69 intermediate the ends thereof, said second tubing concentrically surrounding the walled end of said first piece of tubing.

As shown in Figs. 4 and 5, each of these orifices is preferably in the form of a capillary bore 15 passing through a glass disk 11 which is fused onto one end of a small piece of extension tubing 19, the other end of which is fused around a hole 8| in the wall 65, said disks forming a section of the wall.

To make orifice members suitable for my purpose, I cut a thin slice from a piece of capillary glass tubing, preferably having a bore diameter of less than about 0.0018 inch, said slice being about to of an inch in thickness, and then grind the slice of capillary tubing to a thickness of less than about 0.01 inch. After grinding the slice of capillary tubing to a suitable thickness, the disk is preferably polished to reduce the adsorptive surface thereof, and then secured to the end of extension tubing 19.

An alternate method of producing an orifice member suitable for my purpose is to draw out a metal tube having a fine bore therein, to produce a metallic capillary tube, then cut off a piece of this tube, and grind it down to a suitable thickness. Such a metal disk 'l'l may be secured to one end of each extension tube 19 by means of a suitable cement, such as De Kotinsky cement 82 as shown in Fig. 6.

By grinding such a slice of capillary glass tubing or metal tubing to a suitable thickness, an orifice disk ideally suited for my purposes may be obtained, since by either of these techniques sturdy orifices of very fine bore and low resistance to gas fiow may be produced. However, I prefer to use the glass disk since it is easier to incorporate in my multiple inlet gas passage, and is cleaner since it does not require the use of cement.

The size of the bore is preferably less than about 0.0018 inch, as previously indicated, for if it is much larger, 3, suitable pressure in the sample chamber for achieving linear superposition must be reduced to a point Where a background spectrum produced by gradual release of gas adsorbed on the surface of the sample bottle may become significant in accurate quantitative equation:

Q 334( R3 1 P2 where:

Q=rate of flow in cc. per sec., referred to a unit pressure of one dyne per cm.

R=radius of orifice L=length of orifice dr=density of the component at a pressure of 1 dyne per square centimeter p1=partial pressure of the component in the sample chamber pz=partial pressure of the component in the ionization chamber.

In practice 1 mm. In my method each component will fiow through the orifice at the same rate with which it would fiow if it alone were present, that is at a rate proportional to its partial pressure in the sample chamber and inversely proportional to the square root of its molecular Weight.

Due to the face that light components fiow through the orifice faster than heavy components, the composition of the gas right at the mouth of each orifice on the high pressure side thereof will be slightly deficient in light components, and

will have an excess of heavy components compared with the average ratio of the amounts of said components remaining in the sample chamber. In order to maintain the composition of the sample at the orifice substantially identical with that throughout the rest of the sample chamber, the pressure of the sample is maintained sufficiently low for components to interdiffuse rapidly enough to maintain the mixture at substantially uniform composition throughout the sample chamber. This interdifiusion is aided in the present instance by spacing the orifices 61 apart a distance which is large compared to the radius of the orifices, and placing the connector with the extension tubes 19 with the orifice disks I1 on the end thereof mounted on the high pressure side, and also with the larger cross-section tubing H on the high pressure side.

In an alternate form of my invention, the wall separating the sample chamber I and the ionization chamber 3 may be in the form of a thin disk 83 having a plurality of small bores .85 drilled therein, as shown in Fig. 7.

In still another modification of my invention, a disk containing a plurality of bores therethrough may be mounted at the end of each of the extension tubes 19.

In any casepthe orifice disk which I prefer to use has as small a thickness as is compatible with the disk area so that the disk can sustain a differential pressure of one atmosphere, and the orifice disks will not break in the event that the pressure in the sample chamber is for some reason raised to atmospheric pressure, while the ionization chamber pressure is a high vacuum.

Other forms of my invention utilizing a plurality of concurrently acting inlets through each of which the components of the mixture may flow at independent rates, will readily occur to those skilled in the art.

From the foregoing discussion of my invention it will be apparent that I have provided an improved system for achieving linear superposition in mass spectrometry.

I claim;

1. In the analysis of a gas mixture containing a plurality of components, with a mass spectrometer having an ionization chamber and a sample chamber connected thereto through an inlet system, the improvement which comprises pressure flowing the mixture from the sample chamber into the ionization chamber through a plurality of apertures in said inlet system while maintaining the pressure in the chambers at such values as to flow each component through each aperture at the same rate with which it would flow if it alone were present.

2. In a method of analyzing a gaseous mix ture involving admission of the components of such mixture simultaneously to an ionization zone and at mutually independent rates, ionization of components of the mixture in such zone, the withdrawal of resulting ions from the zone, and maintenance of the pressure in the zone at such a low value that substantially all ions are withdrawn from the zone without collision with molecules of the gaseous mixture, and the determination of the amounts of withdrawn ions of different mass-to-charge ratios, the improvement which comprises simultaneously admittin such components into the ionization chamber through a plurality of inlets.

3. In a method of analyzing a gas mixture with a mass spectrometer, the improvement which comprises simultaneously diffusing each component of a gas mixture from a sample region through a plurality of apertures in parallel into an ionization region and maintaining the pressures of gas in said regions at values such that the difiusion rates of the respective components through each aperture are inversely proportional to the square roots of the molecular weights of the respective components.

4. In analyzing a gas mixture with a mass spectrometer involving passing the mixture from a sample region through an inlet system into an ionization region, the improvement which comprises flowing each component from the sample region into the ionization region through a plurality of apertures in parallel in said inlet system and maintaining the pressure in the sample region greater than the pressure in the ionization region and at a value at which the mean free path of molecules is sufllciently large for the molecules of each component to pass through each said aperture without any substantial number of collisions with other molecules.

5. In a method of analyzing a gaseous mixture involving admission of the mixture from a sample chamber nto an ionization zone, the ionization of components or the mixture in said zone, the withdrawal of the resulting ions from the zone while maintaining the pressure in the zone such that the number of ions derived from the separate components varies in accordance with the partial pressures of the respective components and independently of the partial pressures of other components present, and the determination of the amounts of withdrawn ions of a selected mass-to-charge ratio, the improvement which comprises simultaneously forcing the components of the mixture from the sample region through a plurality of apertures into the ionization zone and maintaining the sample'chamber pressure higher than the ionization zone pressure and at the same time at a pressure so low that the mean free path of molecules of the components of the mixture in the sample chamber is at least as lon as about half of the least cross-sectional dimension of the apertures.

6. In a method of analyzing a gas mixture, the improvement which comprises the steps of simultaneously passing each component of said gas mixture from a high pressure sample region into a low pressure analysis region through each of a plurality of connecting inlets at a rate which is independent of the presence of other gas components present in the mixture, ionizing the molecules of each component in accordance with the partial pressure of said component in said low pressure region, and measuring the respective rates of production of ions of different massto-charge ratios formed.

'7. In a method of analyzing a gas mixture with a mass spectrometer having an ionization chamber and a sample chamber connected thereto, the improvement which comprises introducing said mixture from the sample chamber into the ionization chamber through a plurality of connecting inlets under such conditions of sample chamber pressure and ionization chamber pressure that each component of said mixture flows from the sample chamber into the ionization chamber at a rate dependent on the partial pressure of said component present and independent of the partial pressure of any other component present.

8. A gas inlet system for a mass spectrometer comprising a connector having two portions adapted for connection to a sample chamber and to an ionization chamber respectively, a wall in said connector for separating said chambers, a plurality of extension tubes sealed around holes in said wall, and a flow control member having a small openin therein sealed to each of said tubes at the end thereof remote from said wall to provide a plurality of openings between said chambers.

9. A gas inlet system for a mass spectrometer, comprising a connector including two tubes of diiferent diameter, the tube of smaller diameter having a wall at one end thereof closed except for a plurality of small apertures therethrough, the tube of larger diameter being sealed to said smaller tube intermediate the ends of said smaller tube and extending in the direction or said walled end.

10. In combination with an inlet system as defined in claim 9, an ionization chamber connected to said tube of smaller diameter, and a sample chamber connected to said tube of larger diameter.

11. A gas inlet system for a mass spectrometer comprising a connector having two portions adapted for connection to a sample chamber and to an ionization chamber respectively. and a wall in said connector for separating said chambers and having a plurality of capillary apertures therein in parallel with each other.

12. A gas inlet system for a mass spectrometer comprising a connector having two portions adapted for connection to a sample chamber and to an ionization chamber respectively, and a wall for separating said chambers and having a plurality oi. orifices therethrough in parallel with each other, each of said orifices having a. diameter or the order of 0.0018 inch or less.

13. In combination, an ionization chamber of a mass spectrometer, a sample chamber, and a partition therebetween having a plurality of capillary orifices therethrough in parallel with each other and providing gas flow paths between said chambers. c

14. Apparatus as in claim 13, wherein said orifices are spaced apart a distance which is large compared with the cross-sectional dimensions of said orifices,

15. In combination, a mass spectrometer ionization chamber having an ion beam collimating slit in the wall thereof, a sample chamber, means providing a plurality of paths in parallel through which gas may be concurrently admitted from the sample chamber into the ionization chamber, and a vacuum pumping line connected to provide for evacuation of said ionization chamber primarily through said slit so as to reduce the pressure in the sample chamber, the gas paths and the ionization chamber.

16. In combination, a mass spectrometer ionization chamber having in the wall thereof an aperture through which ionizing particles may be directed and a slit through which ions formed in said ionization chamber may pass, said aperture being of a smaller cross-sectional area than said slit, a. sample chamber, means providing a plurality of paths in parallel through which gas may be concurrently admitted from the sample chamber into the ionization chamber, and a pumping line connected to provide for evacuation of said ionization chamber through said slit so as to reduce the pressure in the sample chamber, the gas paths and the ionization chamber.

17. In combination, a mass spectrometer including an ionization chamber having in the wall thereof a slit through which ions may pass, means for separating and successively detecting ions of different mass-to-charge ratios passing through 

